1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) technique referred to as magnetic resonance (MR) angiography, which acquires images of blood vessels of subjects, based on a magnetic resonance phenomenon occurring in the subjects. More particularly, the present invention is concerned with a magnetic resonance imaging system and magnetic resonance imaging method based on an improved MR angiography technique using a plurality of saturation pulses applied with slice-selective gradient pulses, the saturation pulses being used for separation between arteries and veins, suppression of body motion artifacts, and the like.
2. Description of the Related Art
Magnetic resonance imaging is a technique for magnetically exciting nuclear spins existing in a subject positioned in a static magnetic field by applying a radio-frequency signal with the Larmor frequency, and reconstructing an image using an MR signal induced with the excitation or producing a spectrum of the MR signal.
In the field of MRI, MR angiography techniques for imaging flows of blood within a subject or measuring the flow speed thereof have already been in practical use in medical examination. One of the MR angiography techniques uses a saturation pulse, applied with a slice-selective gradient, causing proton spins of flows of blood to be pre-excited and saturated at the time of acquisition of MR signals.
In the conventional saturation pulse based technique, a signal saturation pulse is applied to a slice positioned upstream or downstream across flows of blood passing through an imaging slice. For example, in the case of imaging the inferior limb, since the directions of flows of blood are opposite to each other between the arteries and veins, a single saturation pulse is applied to a preliminary slice set at an upstream or downstream side to an objective imaging slice, and then echo signals are acquired from the imaging slice using, for example, an FE method. Since the spins (dipoles of magnetization) of the arteries or veins inflowing the imaging slice have already been excited and saturated, they are no longer excited by the succeeding MR data acquisition sequence, thereby reducing the strength of MR signals induced from the blood flow. By contrast, the saturation pulse has not been applied to arteries or arteries inflowing the imaging slice from the opposite side, MR signals of higher strength are acquired from those blood vessels. It may therefore be expected that the arteries and veins be separated from each other on a reconstructed MR image.
However, speeds of blood flows in an imaged region are sometimes very fast or slow (particularly, in the inferior limb, they are slow). In such cases, owing to the fact that a flow-void phenomenon occurs or the inflow of saturated spins becomes extremely slow, the effectiveness of applying the saturation pulse is not enough, thereby providing no images where arteries and veins are distinctly visual-separated from each other.
Additionally, for the limbs, in general, the speeds of pulsated blood flows become remarkably slow depending on distances along peripheral vessels. To be specific, when the sequence incorporating the conventional saturation pulse is used, the blood flows of the inferior limb themselves can hardly be imaged because of the time of flight effect is less. Yet, for example, for knee diseases, to meet a clinical demand that artery-vein clearly separated images are desired is beyond the conventional imaging technique.
Another MR angiography technique different from the sequence using the saturation pulse is under research, which is trying the visual separation between arteries and veins. According to this research, MR contrast medium is injected into a patient and arteries and veins are visually separated based on differences between temporal changes in contrast for arteries and veins.
For MR angiography using MR contrast medium, however, invasiveness due to the injection of MR contrast medium is very large, requiring patients to endure it.
Further in the case of using MR contrast medium, since the actual contrast effect for inferior limb is low, differences in contrast peak times between arteries and veins are small. Thus, for the present, no clear separation images are provided.
Therefore, any conventional MR angiography technique is not suitable for such regions as the limbs where the speeds of blood flows are extremely low. It is almost impossible to provide high-quality artery-vein separated images with non-invasive treatment.